There are 100 trillion good bacteria in your gut. Here's how probiotics and prebiotics work and what you need to know.

Probiotics are the latest craze in the food industry, turning up in everything from pizza to chocolate. They now tally $20 billion in global sales, expanding at 20 to 30% a year. If you're not already consuming them in some form, chances are you will be soon.

"Probiotics are the new vitamins," says Shekhar Challa, MD, a gastroenterologist in Topeka, KS, and the author of Probiotics for Dummies. That's a bold statement, because probiotics are actually live microbes—specifically, beneficial bacteria that promote human health if consumed in large enough quantities. For germophobic Americans, it's a revolutionary concept. But the 100 trillion microbes that live in your large intestine do dozens of good things for you. They process indigestible fibers and help keep bowel function regular. They produce a number of vitamins, including B6, B12, and K2, and aid in the absorption of minerals such as iron, calcium, and magnesium. Equally important, they help fend off bad bacteria such as Salmonella and E. coli, which can cause diarrhea and, in extreme cases, severe anemia, kidney failure, and death.

"The intestines are a war zone, where beneficial and harmful bacteria are fighting to establish predominance," says Venket Rao, PhD, emeritus professor of nutritional sciences at the University of Toronto. The key is for the good guys to outnumber the bad. If you want to give them a competitive edge, a regular supply of probiotics can help.

The payoff can extend well beyond your gut, and your immune system is a prime beneficiary. In a Swedish study of 262 workers, those who took probiotics for 80 days were 42% less likely to take a sick day for an upper respiratory infection or gastrointestinal disease. Regular doses can help reduce vaginal and urinary tract infections. If you're prone to allergies or eczema, probiotics may even help tamp down an overactive immune system. They accomplish all this by producing their own form of antibiotics, blocking pathogens from adhering to the gut, and spurring production of chemical messengers called cytokines, which communicate with the immune system throughout the body. Probiotics may even enhance your mood, thanks to a similar cross talk with the central nervous system.

So the conclusion is simple, right? Take probiotics. Unfortunately, it's not that easy. There are more than 3,000 species of good bacteria in your gut, and each has its own talents. The cultures you're consuming may not be the ones that reduce colds or fight diarrhea. And they have to be handled correctly, so they aren't killed during processing or storage. "No more than 10% of products that claim to be probiotic have been proven in human trials," says Gregor Reid, PhD, chair of human microbiology and probiotics at the Lawson Health Research Institute.

The microbes that turn milk into yogurt and kefir are among the most beneficial, and they seem to thrive in dairy. "Milk contains compounds called oligosaccha-rides [complex carbohydrates] that the bacteria feed on," says Roger A. Clemens, DrPH, adjunct professor at the University of Southern California. Dairy products are also kept chilled, which is important for heat-sensitive organisms, and are only weakly acidic, another plus. (Bacteria can perish in the strongly acidic environment of the stomach, but dairy provides protection.) Just make sure the container says "live and active cultures." Dead bacteria won't help. The more reliable brands tell you which specific bacteria they contain.

Tip: While the strains in most yogurt brands are beneficial, Dannon's Activia yogurt and DanActive drink are among the rare products backed by published, peer-reviewed studies. Kefir contains even more strains than yogurt.

Microbes are responsible for fermentation, turning cabbage into sauerkraut, cucumbers into sour pickles, and soybeans into miso. For thousands of years, fermented foods have been staples of the human diet. But we eat far fewer of these foods today, and when we do, modern processing often kills off the good bacteria. "Stores can't have jars exploding on shelves when bacteria produce gas, so manufacturers pasteurize sauerkraut and pickles," says Mary Ellen Sanders, PhD, executive director of the International Scientific Association for Probiotics and Prebiotics. "Unless you're pulling your pickles out of a crock in a deli or buying them from a small local producer who labels them 'raw fermented,' you're not getting live microbes."

Tip: Make your own, says Sandor Ellix Katz, author of The Art of Fermentation. For sauerkraut, slice cabbage thin, massage it in a bowl with 1 tablespoon of salt to draw out the water, and store it in the brine you've just created. Wait a week or more while it ferments. That's it! Just be sure to keep the kraut completely covered in the brine to prevent rotting caused by bad bacteria. Robyn Jasko, the author of Homesweet Homegrown, recommends a type of jar called Pickl-It (pickl-it.com), which has an airlock that keeps bad bacteria out and good bacteria in.

Probiotics are now being added to lots of unfermented foods too, including cookies, pizza crust, coffee beans, and powdered smoothie mixes. But unless the label promises "live and active cultures," don't count on them—particularly in products that require heating, such as coffee and pizza. High temperatures are likely to destroy the bacteria.

Tip: Chocolate is one product in which added probiotics do well. Attune Foods makes three varieties with beneficial levels of tested strains. Find them in the dairy case at chains such as Safeway.

Throughout most of human history, getting enough good bacteria was no problem. But today we live in a sanitized world. If you need extra help—for example, if you have chronic constipation (63 million Americans do) or you're taking antibiotics, which kill good and bad bacteria alike—probiotic supplements can provide steady, reliable relief. Earlier this year, a study in the Journal of the American Medical Association concluded that patients on antibiotics can reduce the associated risk of diarrhea by 42% if they take probiotics at the same time.

For colonies to thrive, you need to create favorable living conditions for them. One of the best ways to do that is to consume a diet rich in fruits and vegetables. "Processed foods contain preservatives, which are antimicrobial by definition," Dr. Reid says. By contrast, many natural whole foods include prebiotics, or foods that the good microbes themselves feed on—namely, insoluble fibers that people cannot digest but bacteria can. These fibers are in a variety of foods, including onions, bananas, asparagus, leeks, garlic, artichokes, wheat, oats, and soybeans.

Tip: Prebiotics are being added to some packaged foods too. Look for terms like inulin, FOS, GOS, or polydextrose on the ingredient list.

Beyond The Dinner Table

Probiotics aren't just in food anymore. They're turning up in breath mints, mouthwash, hand sanitizer, and cleaning products. Why? If you want to control bad bacteria in your gut, you might also want to keep them in check in your mouth, on your hands, and on kitchen counters. But do these products work? We won't vouch for them all. But in an informal test at Prevention's offices (using a device to measure the biofilm that harbors bad bacteria), the PIP line of probiotic cleaning products outperformed conventional cleaners on a microwave keypad, a computer keyboard, a bathroom floor, and even an employee's hands.

There are so many reasons to exercise—to reduce your blood pressure, build strong bones, and boost mood, to name just a few. But who ever mentions keeping your gut bacteria healthy? "These organisms replicate about every 20 minutes," Dr. Clemens says. Exercise stimulates gut motility, which benefits good bacteria by regularly removing toxins that might harm them. By the way, bowel movements are also a general indicator of how healthy these colonies are. "If you're getting adequate fiber and are physically active, you should have more frequent bowel movements that are larger, softer, and lighter in color," says Dr. Clemens. Surprising fact: Half the volume of your bowel movements is not food waste but bacteria. You have several pounds of these microbes in your gut at any time.

Tip: Thirty to 60 minutes of exercise a day will usually do the trick, Dr. Clemens says. "It's another reason to get out and walk."